[Crustacea • 2016] Tiwaripotamon pluviosum • A New Species of Freshwater Crab of the Genus Tiwaripotamon Bott, 1970 (Brachyura, Potamidae) from northern Vietnam and southern China

A new freshwater crab species of the family Potamidae is described from the boundaries of northern Vietnam and southern China. Tiwaripotamon pluviosum sp. nov. can be distinguished from other species by a suite of morphological characters, including a relatively flat carapace, relatively less slender legs, and distinct dorsal flaps on the male first gonopods. Molecular data using the mitochondrial cytochrome oxidase subunit I (COI) also supports this species as being new.

Etymology. The name “pluviosum” means rainy, which
refers to the specimens being collected and active during
the rainy season.

Ecological notes. This species inhabits limestone mountains
(Fig. 5G, H), and are most active during the rainy season
(personal observation). Similar to T. vixuyenense (cf. Shih &
Do, 2014), specimens of this species were also found crawling
on the leaves and twigs, with one specimen observed being
one metre above the ground at night.

Remarks. This species fits well within the character
descriptions of Tiwaripotamon defined by Ng & Yeo
(2001) due to the squarish third maxilliped ischium, short
third maxilliped exopod with a short flagellum, slender
legs, broadly triangular male abdomen, and an upcurved
G1 terminal segment (Figs. 1–4). This new species can
be separated from other congeners (see below) by the
relatively flat and transverse dorsal surface of the carapace,
proportionately stouter ambulatory legs and distinct dorsal
flap on the G1.

The G1 of this species resembles T. edostilus in the presence
of a dorsal flap, but it is smaller in size in the latter (versus
larger in this species) (Fig. 2; cf. Ng & Yeo, 2001: fig. 5).
The new species can also be separated from T. edostilus
by the flatter and more transverse carapace (ratios of CW:
CL is 1.3 of this species; 1.2 of T. edostilus; cf. Shih &
Do, 2014) and stouter ambulatory legs (see below). The
carapace of the new species is similar with T. annamense
in the morphology of the anterolateral margin. However,
the two can be easily separated by the presence of a dorsal
flap on the G1 in the former (Fig. 2) and the absence of
such in the latter (Ng & Yeo, 2001: fig. 2D–I). The merus
of the fourth ambulatory leg is about 4.8 times longer than
broad for this species, which is shorter than most congeners:
6.5 for T. vixuyenense, 5.1 times for T. vietnamicum, 5.6
times for large T. edostilus specimens and 5.3 times for T.
xiurenense (unpublished data; Shih & Do, 2014), but longer
than T. pingguoense (4.3 times, unpublished data) and T.
annamense (approximately 4.5 times, estimation derived
from dactylus ratio, Ng & Yeo, 2001).